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  • 1
    Digitale Medien
    Digitale Medien
    Small and Medium Rings, 74. Syntheses and Photoelectron Spectra of 7-Azanorbornadiene and Related Compounds An Analysis with Fragment Orbitals (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 791-801 
    Details
    ISSN: 0009-2940
    Schlagwort(e): Lone-pair orientation ; Lone pair interaction ; Precanonical orbitals ; Orbitals, localized ; Calculations, ab initio STO-3G, MNDO, MM2 force field ; PE Spectroscopy ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The He(I) photoelectron spectra (PE) of 7-azanorbornane (5), 7-azanorbornene (6), and 7-azanorbornadiene (7) as well as of related urethanes have been recorded. The syntheses of these bicyclic compounds are described in detail. A most convenient analysis of the PE spectroscopic results is based on the procedure of Heilbronner-Schmelzer on an ab initio STO-3G level. This method allows to construct fragment orbitals elucidating the orientation of the nitrogen lone pair, the through-space interaction with π bonds, and the participation of σ bond orbitals. The result is remarkable: whereas the direct interaction of localized lone-pair and π orbitals is significant in both syn;- and anti-orientation, the interaction of a localized lone pair with a precanonical fragment π orbital is completely different in the two geometries. In 6;-syn a considerable interaction matrix element FantiΨnπ=-0.71 eV comes to the fore, whereas the corresponding parameter in 6;-anti turns out to be almost zero, FantiΨnπ=+ 0.09 eV. Since 6;-syn is calculated to be more stable than 6;-anti, it is this invertomer, 6;-syn, which is likely to be responsible for the main bands in the PE spectrum of 6. The comparatively large experimental split between the first two PE bands of 6 (0.98 eV) is in accord with the 6;-syn geometry and compares well with the calculated band separation of 1.26 eV for 6;-syn.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/cber.19911240420
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  • 2
    Digitale Medien
    Digitale Medien
    Chromophore Systeme, 1. Konformation und Lichtabsorption von Hexa-1,5-dien-3,4-dion (Divinylglyoxal) (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 821-826 
    Details
    ISSN: 0009-2940
    Schlagwort(e): α-Diketones, unsaturated ; Conformers, stable ; UV-Vis transition energies ; PE Spectroscopy ; 13C-NMR chemical shifts, correlation with dihedral angles CO/CO ; Calculations, STO-3G, MMP1 ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Chromophoric Systems, 1.  -  Conformation and Absorption of Light in Hexa-1,5-diene-3,4-dione (Divinylglyoxal)Divinylglyoxal (1) is investigated by several spectroscopic and theoretical methods in order to set up a correlation between the CO/CO dihedral angle and the transition energies in unsaturated α-diketones. According to the experimental (UV, IR, 13C-NMR, PE spectroscopy and dipole moment) and theoretical (ab initio and molecular mechanics) results, 1 exists predominantly as a C2h conformer (Θco/co=180°, ΘVinyl/co=0°). The dependence of transition energies on CO/CO dihedral angles is similar in shape to that for saturated α-diketones, but is significantly shifted to lower transition energies. 13C chemical shifts respond sensitively to CO/CO dihedral angles, and the ab initio calculated curve is supported by a few experimental results.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/cber.19911240423
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  • 3
    Digitale Medien
    Digitale Medien
    PE-spectroscopic investigation of nonconjugated π,π interactions in (seco)dodecahedradienes  -  on the route to an experimental ethylene/ethylene cycloaddition reaction coordinate (1995)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 2019-2025 
    Details
    ISSN: 0947-3440
    Schlagwort(e): (Seco)dodecahedradienes ; PE spectroscopy ; π,π Interaction, nonconjugated, through-space, through-bond ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Nature and extent of nonconjugated π,π interactions in (bis)-secododecahedradienes and (pentagonal, non-pentagonal) dodecahedradienes with transannular π,π distances ranging from ca. 2.8 to ca. 3.5 Å - representing well defined points on the D2h [π2 + π2] reaction coordinate - were studied by PE spectroscopy. The π,π splitting of 0.68 eV measured at the largest π,π distance of ca. 3.5 Å (1,16-dodecahedradiene 3a) was attributed to ca. 50% each of through-space and through-bond conjugation (analysis of precanonical MOs).
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.1995199511283
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  • 4
    Digitale Medien
    Digitale Medien
    Vinylogous β-Carotenes: Generation, Storage, and Delocalization of Charge in Carotenoids (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 2205-2213 
    Details
    ISSN: 0947-3440
    Schlagwort(e): Carotenoids ; C26—C70 β-carotenes ; Cyclic voltammetry ; Photoelectron spectroscopy ; Semiempirical calculations ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In this paper, we report on the synthesis of β-carotenes of variable chain length with between 5 and 23 double bonds (1-7). These oligoenes were prepared by McMurry and Wittig reactions. The tetradecapreno-β-carotene 7 would seem to be the longest β-carotene yet reported. Furthermore, we investigated the electronic properties using cyclic voltammetry and photoelectron spectroscopy (UPS) to generate open-shell and closed-shell ions of carotenoids in solution and in the solid state, respectively. With increasing chain length (≥ 11 double bonds), even the generation of tetracations and tetraanions could be observed by cyclic voltammetry. Extending the number of conjugated bonds causes the potentials to converge to limiting values. All electron-transfer processes occur in one-electron steps, which are close to each other in pairs. The potential difference between the first oxidation potential and the first reduction potential is a linear function of the reciprocal chain length. Despite the different techniques used (CV and UPS) and the different condensed phases, there is an excellent correspondence between the energies of the radical cation states generated by the two methods. This shows that the radical cation formation is principally determined by the chain length. The structures of the ion states were investigated using semiempirical methods at the NDDO level. Charge delocalization and bond relaxation are not identical and do not utilize the same number or the same kind of atoms. It can be shown that from the length of 20 double bonds onwards, the effective conjugation length for doubly-charged cations converges slowly to a limiting value.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.199719971106
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  • 5
    Digitale Medien
    Digitale Medien
    Singlet Oxygen Quenching Abilities of CarotenoidsDedicated to Professor Waldemar Adam on the occasion of his 60th birthday. (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 1887-1893 
    Details
    ISSN: 0947-3440
    Schlagwort(e): Carotenoids ; Isonorastacene synthesis ; Singlet oxygen ; Effective chain length ; Second-order quenching rate constants ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The bimolecular rate constants kq for quenching of singlet oxygen (1Δg state) by 26 different natural and novel synthetic carotenoids were determined at 37 °C in a mixture of chloroform and ethanol. The steady-state technique used involves the generation of 1O2 by thermal decomposition of disodium 3,3′-naphtalene-1,4-diyl-dipropionate endoperoxide (NDPO2) and the detection of its luminescence intensity at 1270 nm. Excitation energies (π,π*, 11Ag → 11Bu) and absorption maxima (430-590 nm) vary in the broadest range. Deeply coloured blue carotenoids are also included in the studies for the first time. An empirical correlation between the π,π* (11Ag → 11Bu) excitation energy and carotenoid structure (effective chain length Neff) was found: E(S) = 12642 cm-1 + 92027 cm-1 × 1/Neff. The quenching ability of the investigated carotenoids depends on the excitation energy of their transition at long wavelengths in a characteristic way showing as limiting factors either the thermal Arrhenius activation or the diffusion-controlled rate. This dependence and the suspected relationship between singlet E(S) and triplet E(T) energies, respectively, are discussed.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.199719970913
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  • 6
    Digitale Medien
    Digitale Medien
    Coupling of Chromophores: Carotenoids and Photoactive Diarylethenes - Photoreactivity versus Radiationless Deactivation (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 2333-2338 
    Details
    ISSN: 1434-193X
    Schlagwort(e): Photochromism ; Optical switches ; Dithienylethenes ; Carotenoids ; Valence isomerization ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: New photochromic carotenoid-like chromophores containing the dithienylperfluorocyclopentene fragment have been synthesized and characterized by UV/Vis spectroscopy. The quantum yields of the photochromic forward and back reactions of these compounds have been determined and are found to decrease sharply with increasing chain length of the substituent. This decrease in quantum yield can be rationalized in terms of a decrease of the excitation density at the central photoreactive unit and of the excited state lifetime, which is known to shorten in carotenoids with increasing chain length.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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